Cross sections and NO product state distributions resulting from substrate mediated photodissociation of NO₂ adsorbed on Pd(111)

Ultraviolet irradiation of NO2 adsorbed on top of a NO saturated Pd(111) surface causes the photodissociation of NO2/N2O4 and results in the desorption of NO molecules. This process has been studied using excitation energies between 3.5 and 6.4 eV. At a photon energy of 6.4 eV, a cross section of 3×10−18 cm2 is found. Using laser‐induced fluorescence to detect the desorbed NO molecules, fully state‐resolved data detailing the energy channeling into different degrees of freedom has been obtained. Two desorption channels are found, one characterized by nonthermal state populations, and one showing accommodation to the surface. The yield of the fast channel shows a marked increase above 4 eV photon energy. The slow channel is interpreted as being due to NO molecules which, after formation, undergo a trapping–desorption process. A polarization experiment indicates that the photodissociation is initiated by excitation of metal electrons rather than direct absorption by the adsorbate

Self-organized stable pacemakers near the onset of birhythmicity

General amplitude equations for reaction-diffusion systems near to the soft onset of birhythmicity described by a supercritical pitchfork-Hopf bifurcation are derived. Using these equations and applying singular perturbation theory, we show that stable autonomous pacemakers represent a generic kind of spatiotemporal patterns in such systems. This is verified by numerical simulations, which also show the existence of breathing and swinging pacemaker solutions. The drift of self-organized pacemakers in media with spatial parameter gradients is analytically and numerically investigated.Comment: 4 pages, 4 figure

Size-Dependent Transition to High-Dimensional Chaotic Dynamics in a Two-Dimensional Excitable Medium

The spatiotemporal dynamics of an excitable medium with multiple spiral defects is shown to vary smoothly with system size from short-lived transients for small systems to extensive chaos for large systems. A comparison of the Lyapunov dimension density with the average spiral defect density suggests an average dimension per spiral defect varying between three and seven. We discuss some implications of these results for experimental studies of excitable media.Comment: 5 pages, Latex, 4 figure

Disorder-Induced Anomalous Kinetics in the A+A→∅A+A \to \emptyset Reaction

We address the two-dimensional bimolecular annihilation reaction $A + A \to \emptyset$ in the presence of random impurities. Impurities with sufficiently long-ranged interaction energies are known to lead to anomalous diffusion, $\sim t^{1-\delta}$, in the absence of reaction. Applying renormalization group theory to a field theoretic description of this reaction, we find that this disorder also leads to anomalous kinetics in the long time limit: $c(t) \sim t^{\delta -1}$. This kinetics results because the disorder forces the system into the (sub)diffusion controlled regime, in which the kinetics must become anomalous.Comment: REVTEX, 4 pages. More discussion added. To appear in Phys. Rev. E (March, 1998

The scanning photoemission microscope: a novel tool in surface science

After a brief review of scanning photoemission microscopes (SPM) and related instruments, the present state of our SPM, that utilizes a deuterium discharge lamp, is outlined. Earlier published data are reproduced to give a complete overview of all features of the instrument. Several examples illustrate its applicability to the study of physical and chemical processes at metal surfaces